The goal of this project is to contribute to the development of therapies to treat the CNS complications of AIDS through basic study of lentiviral replication in the brain. The human immunodeficiency virus (HIV) frequently causes serious neurological abnormalities in infected individuals. The basis of the neurological dysfunction seen in HIV infection remains largely unknown. The available evidence suggests that active replication of virus, and perhaps production of viral proteins in the brain, indirectly causes neuronal injury and dysfunction. However, HIV infection of the nervous system has proven difficult to investigate. Animal models are lacking because HIV infection is essentially limited to humans. The human brain is inaccessible except for studies using autopsy materials. Human immune cells are easily harvested from adult humans and can be maintained in culture for study. The equivalent in vitro studies of the central nervous system require human fetal tissue that is far more difficult to obtain. In this application, a novel model system would be studied to determine the effects of visna viral replication on the nervous system and neuronal cells in tissue culture from sheep, the natural host of visna. Visna virus and HIV are both retroviruses of the lentivirus subfamily which infect the same target cells in the brain. The two viruses probably cause damage to the central nervous system during infection through common mechanisms. This laboratory has recently developed techniques to culture primary neurons from fetal sheep, and have begun to characterize the cultures and their infection by visna virus. Neurons are maintained in contact with glia and develop extensive processes and synaptic contacts during weeks to months in culture. These sheep cultures can be infected with visna virus. This system provides the unique opportunity to study the acute and chronic effects of lentivirus infection on neurons and glia derived from the natural host. The hypothesis is that viral replication in non-neuronal cells indirectly causes neuronal dysfunction. The Specific Aims are: 1) the fate of cell types in the cultures infected by visna virus will be determined. The identity and number of infected cells will be established. The prediction is that in chronically infected cultures a relatively small number of infected macrophages harbor virus and are responsible for indirect effects on neurons; 2) how neurons control the replication of visna virus in CNS cultures will be determined. Preliminary results demonstrate that viral replication is powerfully inhibited by neurons. It is suspected that a signal from neurons may suppress virus gene expression in glia, blocking replication; 3) mechanisms by which infected glia or macrophages in co-cultures with neurons enhance neuronal injury will be investigated. The effect of infection and viral proteins on neurotoxicity and expression of stress proteins will be determined. In this way the direct toxic effects of viral proteins and the indirect effects of immune mediators released by infected cells will be distinguished. These studies may ultimately produce new approaches to preventing the CNS complications of HIV infection.